Codec tests and documentation needs to be updated

[sethmlarson]

Current tests and documentation in the README references the codec for this library as "idna" instead of "idna2008", should be updated. This value was changed in #131

This also makes me wonder if the test suite needs to test specific IDNA 2008 functionality since tests don't appear to be failing? cc @kjd

[kjd]

I am exploring ways of addressing #128 and I am not sure this is the right approach yet. But yes, before this is pushed in a subsequent release it needs to line up with the documentation, and some covering tests need to be added.